The present invention relates to data communication systems and, in particular, to channel coding and equalization in such systems.
Much attention has been focused in recent years on signal-space codes which provide so-called coding gain. Prominent among these are the so-called "trellis" codes described in such papers as G. Ungerboeck, "Channel Coding with Multilevel/Phase Signals," IEEE Trans. Information Theory, IT-28, 1982, pages 55-67; A. R. Calderbank and N. J. A. Sloane, "A New Family of Codes for Dial-Up Voice Lines," Procceedings of the IEEE Global Telecomm. Conf., November, 1984, pages 20.2.1-20.2.4; A. R. Calderbank and N. J. A. Sloane, "Four-Dimensional Modulation With an Eight-State Trellis Code," AT&T Technical Journal, Vol. 64, No. 5, May-June, 1985, pages 1005-1018; A. R. Calderbank and N. J. A. Sloane, "An Eight-Dimensional Trellis Code," Proceedings of the IEEE, Vol. 74, No. 5, May 1986, pages 757-759; and L. -F. Wei, "Rotationally Invariant Convolutional Channel Coding with Expanded Signal Space--Part I: 180 Degrees and Part II: Nonlinear Codes," IEEE J. Select. Areas Commun., Vol. SAC-2, September, 1984, pages 659-686, all of which are hereby incorporated by reference. Commercial use of these codes has, for the most part, been concentrated in voiceband data sets and other carrier data communication systems.
Furthermore, a great deal of research has been done on decision feedback equalization (DFE). Specifically, DFE was first explored theoretically in M. E. Austin, "Decision Feedback Equalization for Digital Communication over Dispersive Channels," M.I.T. R.L.E. Technical Report 461, August, 1967. The theory of DFE was subsequently fully dissected in R. Price, "Nonlinearly Feedback-Equalized PAM vs. Capacity for Noisy Linear Channels," Rec. IEEE Int. Conf. Commun., Philadelphia, Pa., June 19-21, 1972, pages 22.12-22.17; and J. Salz, "Optimum Mean-Square Decision Feedback Equalization," AT&T Bell System Technical Journal Vol. 52, No. 8. October, 1973, pages 1341-1373. The application of DFE for voice-band data transmission was evaluated in terms of its performance as in D. D. Falconer, "Application of Passband Decision Feedback Equalization in Two-Dimensional Data Communication Systems," IEEE Trans. Commun., Vol. Com-24, No. 10, October, 1976, pages 1159-1166; D. D. Falconer and F. R. Magee, "Evaluation of Decision Feedback Equalization and Viterbi Algorithm Detection for Voiceband Data Transmission - Part I," IEEE Trans. Commun., Vol. Com-24, No. 10, October, 1976, pages 1130-1139; and D. D. Falconer and F. R. Magee, "Evaluation of Decision Feedback Equalization and Viterbi Algorithm Detection for Voiceband Data Transmission - Part II," IEEE Trans. Commun., Vol. Com-24, No. 11, November, 1976, pages 1238-1245. Performance comparisons between DFE and other schemes for data transmission over, for example, a cross-talk dominated channel were made in papers such as N. A. Zervos and I. Kalet, "Optimized Decision Feedback Equalization Versus Optimized Orthogonal Frequency Division Multiplexing for High-Speed Data Transmission Over the Local Cable Network," Proceedings of the Int. Conf. on Communications, June, 1989, Boston, Mass. All of the aforementioned publications are hereby incorporated by reference.
The technique of DFE focuses on correcting intersymbol interference that impairs data signals when transmitted over, for example, voice-band and cable channels. Specifically, DFE combines the use of some linear equalization--needed to equalize the so-called precursors in the line signal--with a filtering of priorly formed data decisions--used to equalize the so-called post-cursors. The intersymbol interference problem is exacerbated when the data signals are transmitted over the so-called dispersive bandlimited channels at a relatively high rate. DFE, unlike conventional linear equalization, corrects such a problem without incurring a significant enhancement of the noise present in the received signals.
An example of the aforementioned dispersive bandlimited channel is a two-wire pair or "local loop" of a telephone cable network that connects customer premises to a telephone central office. Thus, the DFE technique would be particularly useful for implementing Integrated Services Digital Network (ISDN) service which will demand high signal rates on such local loops in the coming decade. In particular, ISDN will provide, using a) a unified addressing and signaling scheme and b) a single physical point of access, the capabilities that are now provided by a host of separate networks, such as voice, circuit-data, packet-data, telex, private-line networks, etc. Central to the implementation of ISDN is the notion of completing the digitalization of the telephone network by providing a customer with duplex, i.e., simultaneous two-directional, digital transmission capability to the central office over the single two-wire pair or local loop at a distance of up to 18 kft at speeds ranging from the so-called ISDN "basic" (2B+D) rate (with framing, maintenance and control bits) of 160 kb/s up to the so called "primary" (23B+D) rate (again with framing, maintenance and control bits) of 1.544 Mb/s and even beyond.
A conventional equalizer that implements DFE structurally comprises a feedforward section normally made up of a feedforward (FF) linear filter and a decision feedback section normally made up of a feedback (FB) linear filter. Theoretically, the FF and FB linear filters can be of infinite order. This being so, the equalization can be optimized based on the criteria of (1) the so-called probability of error and (2) the so-called mean-squared error. That is, for a given channel of fixed characteristics, one can theoretically determine two individual sets of coefficients associated with the FF and FB linear filters to respectively minimize (1) and (2). Assuming a high signal-to-noise ratio (SNR) in a particular communication system, those two individual sets of coefficients become identical.
Nevertheless, in practice, one cannot implement linear filters of infinite order which conceivably contain an infinite number of coefficients, not to mention to update such a number of coefficients to adapt to a real channel whose characteristics, of course, are not fixed in time. In the prior art, decision feedback equalizers implement finite-order FF and FB linear filters, some or all of whose coefficients can dynamically be updated in accordance with the changing channel characteristics. Examples of these equalizers are one comprised of an FF filter with fixed coefficients and an FB filter with coefficients updatable as described in R. B. Blake, et al., "An ISDN 2B+D Basic Access Transmission System," International Symposium on Subscriber Loop Service, Sept. 29-Oct. 3, Tokyo, Japan, hereby incorporated by reference, and another one comprised of FF and FB filters, all of whose coefficients are updatable as described in K. Watanabe, K. Inoue, Y. Sato, "A 4800 BPS Microprocessor Data Modem," Data National Telecommunication Conference, 47.6-252-47.6-256, also hereby incorporated by reference.